Introduction: Wild beaches are uncontrolled water areas that attract many bathers in summer. Potential chemical or microbiological contamination of natural bathing waters is a threat to human health. Fecal contamination of bathing waters is determined by microbiological parameters such as the presence of enterococci and E. coli bacteria. Purpose: We determined the microbiological quality of bathing waters in Podpeč and Rakiško, which are not on the list of bathing waters, to verify the suitability of two methods for determining the presence of E. coli bacteria in surface waters, and to show that potentially pathogenic E. coli strains are present in both lakes. Methods: A total of 48 surface water samples were collected in all four seasons. The concentration of nitrates, nitrites, ammonium, pH values and temeprature of the water and air temperature were meaured. The presence and number of coliform bacteria, including E. coli and enterococci, were detected on two chromogenic microbiological selective media and at different incubation temperatures. We performed biochemical identification of bacterial strains of E. coli and determined genotypically pathogenic strains of E. coli. Results: Bacterial counts of E. coli and enterococci differed in water samples from the two lakes, but total coliform counts did not. In some seasons we found characteristic differences in the number of bacteria studied. Lake Podpeč was classified in the class with poor bathing water quality, while Lake Rakiško was classified in the class with excellent quality. The media Coliforms Chromogenic Agar at an incubation temperature of 36 °C and Tryptone Bile X-glucuronide Agar at an incubation temperature of 44 °C are suitable for the detection of E. coli bacteria by the filtration method in surface waters, as we found no significant differences between the results in the number of bacterial colonies of E. coli. Potentially pathogenic, mostly verotoxic E. coli strains, were present in both lakes. Discussion and conclusion: Bathing water control is extremely important in certain bathing areas, beaches, as well as in uncontrolled waters that are nevertheless frequently visited by bathers. The methods we used are suitable for monitoring the number of bacterial colonies of E. coli in surface waters, but it would be good to investigate the suitability for counting E. coli colonies at elevated incubation temperature for the Coliforms Chromogenic Agar medium.
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